Highlight differences when changing features

When adding or removing a selected feature, it may not be obvious what parts of the resulting code has changed. This is especially the case for Cairo because there is a lot of code that requires scrolling to view.

Enhance the UI so that differences are visibly highlighted when changing a feature.

For example, if the user starts with:

then enables Mintable like the below:

, this should cause the following sections to be highlighted:

import "@openzeppelin/contracts/access/Ownable.sol";

and

    function mint(address to, uint256 amount) public onlyOwner {
        _mint(to, amount);
    }

Start by reproducing the feature toggle in the contracts wizard UI and compare the generated code before and after enabling Mintable. The work is done when sections added or removed by a feature, such as the Ownable import and mint function, are visibly highlighted.